refactor: separate log timestamp from message as structured LogEntry

Introduced models.LogEntry{Message, CreatedAt} to replace raw strings
in the log pipeline. Timestamps are now formatted at render time, not
baked into stored messages.

- Engine: appendLog stores LogEntry with time.Now()
- Store: LoadLogs returns []LogEntry, selects created_at from DB
- Store: strips legacy [HH:MM] prefix from pre-refactor DB entries
- TUI: sidebar shows "MM/DD HH:MM" from CreatedAt
- TUI: full log view shows "MM/DD HH:MM" from CreatedAt
- SaveLog still receives plain message string (DB handles timestamp)

internal/models/models.go

@@ -86,6 +86,11 @@ type ProbeNode struct {
 	Version  string
 }
 
+type LogEntry struct {
+	Message   string
+	CreatedAt time.Time
+}
+
 // AlertHealthRecord is the persisted send health of an alert channel. It lets the
 // "last sent" / health indicators survive restarts instead of resetting to "never".
 type AlertHealthRecord struct {